System and apparatus for control and optimization of filtration process

ABSTRACT

A process control apparatus and system for dewatering a solids ladened slurry of water and waste water embodying one or more of four primary process control techniques, depending upon the particular process application. 
     The four process control techniques are (1) monitoring of the instantaneous solids concentration increase in the dewatering unit, (2) measurement of the rate of change of the solids concentration increase with respect to time, (3) controlled pumping to produce a substantially constant flow rate in the effluent from the dewatering unit until the terminal dewatering pressure is reached, and (4) variation of the amount of conditioning agents added to the influent slurry in response to one or more control signals derived from other components of the dewatering system and apparatus.

This is a division of application Ser. No. 877,168, filed Feb. 13, 1978,now U.S. Pat. No. 4,151,080.

BACKGROUND OF THE INVENTION

The present invention is related to and is specifically directed to aprocess control in an apparatus and in a system for separating aliquid/solid slurry.

The present invention relates to equipment of the kind in which thesolids are removed from the slurry and are concentrated in theseparating equipment.

In equipment of this kind, an influent solids ladened slurry is pumped,or otherwise fed, to an inlet of the dewatering equipment. These solidsare removed from the slurry in the dewatering equipment and areconcentrated within the dewatering equipment so that a relativelysolids-free effluent flows from the outlet of the dewatering equipment.

The present invention relates to and is used in both continuous flow andbatch flow dewatering processes.

Thus, the present invention relates to continuous flow dewateringprocesses such as belt presses, centrifuges, vacuum filters, sandfilters and other liquid/solids separation equipment.

The present invention also relates to batch flow or cycle liquid/soldseparation processes such as those utilizing a filter press.

However, in most batch flow processes some control of the pumping isusually desirable, to maximize the build-up of the cake solids in theshortest period of time or to avoid choking of the filter cloth.

A programmed control of the pump pressure build-up to avoid choking ofthe filter cloth is disclosed in detail in co-pending U.S. applicationSer. No. 836,506 filed Sept. 26, 1977, by Zuckerman et al., and entitled"Slurry Dewatering Apparatus and Control System" and assigned to thesame assignee as the assignee of this present application.

The present invention also relates to dewatering processes involvingdifferent types of liquid/solid slurries or sludges, such as relativelyincompressible sludges and relatively compressible sludges. The presentinvention also relates to slurries which may require the addition ofconditioning agents to enhance the separation process (such as organicmunicipal sewage sludges) and to other types of slurries that generallydo not require the addition of conditioning agents (such as inorganiccoal slurries and paper mill primary sludges).

There is a need for process control in both continuous flow and in batchflow dewatering applications. In the prior art much of the operation ofthe dewatering equipment is based on pilot studies or preset operatingparameter set from similar applications or fixed during start-up whichdo not take into account or provide for changing conditions occurring inthe actual operation of dewatering processes.

As a result, the prior art separation methods and apparatus havepresented a number of problems.

In some cases the prior art methods and apparatus have not produced aproduct to meet certain requirements, for example, a high enough percentof solids concentration to meet requirements for land fill or forincineration of the filtered solids without the need for supplementalfuel.

The prior art methods and apparatus in some cases have not producedmaximum yields with a desired minimum requirement of power.

The prior art methods and apparatus in some cases also have not producedthe throughput capacity required.

Some of the problems associated with prior art separation methods andapparatus can perhaps best be illustrated by considering a specificbatch flow dewatering method and apparatus using a filter press.

The use of a filter press for dewatering slurries is a well knownmethod.

A filter press dewatering system, generally speaking, consists of afilter press, a pumping system to feed slurries to the filter press andslurry conditioning system.

The major components of a filter press are the filter press frame,filter cloth, filter plates and the hydraulic closing system.

The filter plates are so designed as to form a series of cloth coveredchambers. A solids ladened liquid or sludge is pumped to one side of thecloth, and relatively solid-free liquid is removed from the other sideof the cloth.

The hydraulic cylinder and frame are so constructed as to keep theplates in a closed filter pack which allows the dewatering of the sludgeto occur.

Dewatering in a filter press of this kind is accomplished by pumping theslurry under pressure to one side of the cloth and by removing filtratefrom the other side of the cloth at essentially atmospheric pressure.The pressure differential causes the slurries to physically give upwater.

Typical terminal sludge feed pressures are in a range between 7 and 15atmospheres.

During the filtration cycle dewatered sludge builds up on the sludgeside of the cloth until a cake is formed between adjacent filter plates.

Typical filter press pumping systems consist of piston pumps working incombination with an equalization tank, or high capacity centrifugalpumps working in combination with piston pumps, or piston or diaphragmpumps designed to pump at a maximum rate of flow that the filter presscan receive.

The conventional prior art is to terminate the filter press filtrationprocess under one of the following methods:

(a) A fixed, filtration cycle time based on a pre-set timer. Typicalfiltration times are 30 minutes to 3 hours.

(b) Measure the flow of filtrate from the filter press and terminate thefilter press operation when the filtrate flow falls below apre-determined rate (for example, a certain number of gallons per minuteper square foot of filtration area times the number of chambers).Typically, the minimum filtrate flow is calculated at 0.01 to 0.05gpm/square foot of filtration area in the filter press.

The objective of filtration is to produce a slurry cake which issubstantially dry.

In the prior art it has been possible to determine the dryness of thefilter cake (at various intermediate points during the cycle) only afterthe filtration cycle has been completed. Thus, typically in the priorart, a pilot cycle was run and the resulting cake was then burned in anoven to determine the percent of solids as related to the percent ofmoisture. Then, knowing the final percent solids concentration of thecake and knowing the total overall time of the cycle, intermediate cakeconcentration values could be calculated for various times in the cycleon a time ratio or effluent volume flow ratio basis.

This method of calculating intermediate solids concentration values atvarious points of time during the cycle has a number of drawbacks.

It is (as pointed out above) an after the fact type of determination.

It also rested on a number of assumptions. For example, it assumed aconstant sludge feed concentration, but in actual process applicationsthe percent of solids in the influent slurry can and do varysubstantially.

The prior art also assumed a specific gravity of 1.0 for the sludge, butin actual practice the specific gravity is always greater than 1.0 andvaries.

Also, as a practical matter, the nature of the solids in the influentslurry can change in process; and this affects the dewaterability of theslurry. For example, an increase in the amount of incompressible sand orgrit in municipal sewage, or a change in the nature of the sludgeitself, such as the sludge becoming septic and thus more difficult todewater, can have a substantial impact on the instantaneous increase insolids concentration in the filter press at any particular point in timeor over any particular interval in time.

Thus, prior art method for terminating the filtration process in afilter press (on either a fixed, pre-determined time basis or on themeasurement of flow of filtrate basis) could result in the actual cakesolids concentration varying substantially from that produced during thepilot run or from the preset parameters from similar applications orfixed during start-up.

SUMMARY OF THE PRESENT INVENTION

It is a primary object of the present invention to make qualitative andquantitative measurements of parameters around the liquid/solidseparation unit and to utilize these parameters to indicate and tocontrol the actual in process performance of the separation apparatusand system.

The present invention provides four primary parameters of control forseparation processes and equipment. These four parameters are:

(1) Continuous monitoring of the actual increase of solids concentrationwithin the dewatering unit,

(2) Measuring the rate of change of solids concentration increase withrespect to time within a filter press,

(3) Controlled pumping of the influent slurry to a filter press tomaintain a substantially constant flow rate in the effluent from afilter press, and

(4) Control of the amounts of conditioning agents added to the influentslurry in response to one or more other conditions of the separationprocess and equipment.

Depending upon the particular separation process involved and the natureof the slurry, one or more of these four primary parameters may be usedseparately, or in combination with one or more other parameters, forprocess control purposes.

It is a specific objective of the present invention to maximize theyield of a filter press by minimizing the operating time to reach a cakesolids objective.

It is another specific objective of the present invention to provide theflexibility to produce optimum yield and to be responsive to a range ofinput solids concentrations in the influent slurry.

It is another important objective of the present invention to minimizepower requirements in a filter press by reaching a desired cake solidssolids objective in a minimum of time.

It is a still further objective of the present invention to producemaximum utilization of available equipment, by improving product yield,thus reducing the need for capital expenditure for additional equipment.

In a batch process dewatering unit, such as a filter press, the durationof the pumping and dewatering cycle in accordance with the presentinvention is preferably controlled in response to monitoring of the cakesolids increase within the filter press. In one specific embodiment ofthe present invention the cycle is terminated when the desiredconcentration of cake solids are obtained.

The continuous readout of the actual instantaneous cake solids increasein accordance with the present invention is combined, in one embodimentof the invention, with a regulation of a conditioning system to addconditioning agents to the influent slurry in increasing or decreasingamounts depending on whether the slurry separates worse than or betterthan designed.

In processes in which the separation unit is a filter press, themeasurement of the rate of change of solids concentration increase withrespect to time within the filter press (hereinafter referred to as theResource Factor readout) is also combined with cake solids monitoring(as described above). This combination of controls terminates the cyclewhen a specified minimum Resource Factor (that is, a minimum rate ofchange of cake solids increase with respect to time) is achieved, eventhough the desired amount of cake solids has not been achieved. ThisResource Factor override termination avoids operation in the decreasingreturns range where energy is wasted. Further the Resource Factor methodshortens cycle time to permit increased cycles and increased throughputof the system.

For filter press process applications the Resource Factor control can beused separately and without the cake solids monitoring but for manypractical applications the Resource Factor control can be used inconjunction with the cake solids monitor.

In one mode of operation the present invention uses the Resource Factorto terminate the cycle at a preset minimum Resource Factor.

The minimum Resource Factor control terminates the cycle for a conditionin which the dewatering is worse than designed. In this event the systemgets less cake solids than designed, but it conserves energy andmaintained desired throughput capacity.

For a sludge better than designed the cycle is terminated when the cakesolids reaches the preset value and the equipment is free to dewateradditional sludge.

As noted above, the present invention also uses a combination of cakesolids monitoring and Resource Factor monitoring. The cake solidsmonitoring provides the controlling signal if the rate of separation isbetter than designed. The Resource Factor monitoring provides theterminating signal if the rate of separation is worse than designed.

The set point of the minimum Resource Factor and the set point of thegoverning cake solids can also be varied, in accordance with the presentinvention, and while separation is in process.

With some separation equipment, such as a belt press, vacuum filter,centrifuge, or certain other liquid-solid separation units, theseparation process is achieved with a continuous flow through theseparation unit. Since this type of separation system is not a batchprocess, but is instead a continuous flow process, the controlledpumping system is generally not critical. In process applications ofthis kind the present invention provides a continuous monitoring of theresulting filtered solids concentration. However, there are numerousmachine parameter such as belt tension for belt press, submergence forvacuum filters and pool depth or differential schroll speed forcentrifuge which can be varied to produce changes in performance. Thesignal provided by the monitoring of the filtered-solids concentrationin the separation unit is used, in accordance with one embodiment of thepresent invention, to control the conditioning or one or more of theabove identified machine parameters of the influent liquid/solid mixtureof the slurry to either increase the amount of conditioning agents addedand/or vary the machine parameter if the slurry separates worse thandesigned or to decrease the amount of conditioning agents added if theslurry separates better than designed.

The present invention provides a number of controls for varying theamount of conditioning agents added to the influent slurry to enhancethe separation of the liquid phase from the solids phase by structuringthe solids phase to permit ease of dewatering.

In one specific embodiment the amount of conditioning agent or agentsadded is varied in response to changes in the percent solids in theinfluent slurry.

The conditioning control and cake solids monitoring are also used incombination in another embodiment of the present invention for twoconditions of separation. If the actual separation process is betterthan designed, then the cake solids monitoring signal causes theconditioning system to add less conditioning agent. If the separationprocess is worse than designed, then the cake solids monitoring signalprovides for the addition of greater conditioning agents.

If the slurry does not separate as well as designed (for example, ifsludge goes septic) then this combination system is used to maintain thethroughput rate. (In this event, over conditioning is used to maintainthe desired throughput.)

Alternatively, the Resource Factor override is used to permit lower cakesolids in the case of a filter press dewatering unit. In addition, theResource Factor set point can be lowered to get more cycles per day asdescribed above.

The override can thus be either from cake solids monitoring or fromResource Factor monitoring to override the set point of the conditioningsystem (as responsive to percent of solids in the influent slurry) toget back to the throughput rate desired.

In another embodiment, the present invention uses a pumping system whichis related to the type of slurry. For certain slurries some of which areincluded in a class as incompressible, and the dewatering apparatusincorporates a filter press, then the present invention utilizes apumping system control which varies the pumping pressure build-up tomaintain a constant effluent flow rate out of the dewatering unit duringthe build-to-maximum pumping pressure; and this constant effluent flowrate maximizes the efficiency of build up or increase of cake solids inthe filter press.

For certain other sludges some of which are included in a class ascompressible sludge, the pumping system uses a programmed pressurebuild-up of the pumping pressure into the filter press (such asdescribed in more detail in pending U.S. application Ser. No. 836,506filed Sept. 26, 1977 by Zuckerman et al. and assigned to the sameassignee as the assignee of this application as noted above). Thisprogrammed pressure build-up avoids a blinding or choking of the filtercloth in a filter press and also provides a faster build-up of cakesolids within the filter press than would be obtained with aconventional fast pressure build-up of filter press inlet pressure.

A process control apparatus and system for separating a solids ladenedslurry and incorporating the structure and techniques described aboveand effective to function as described above constitute further specificobjects of this invention.

Other and further objects of the present invention will be apparent fromthe following description and claims and are illustrated in theaccompanying drawings which, by way of illustration, show preferredembodiments of the present invention and the principles thereof and whatare now considered to be the best modes contemplated for applying theseprinciples. Other embodiments of the invention embodying the same orequivalent principles may be used and structural changes may be made asdesired by those skilled in the art without departing from the presentinvention and the purview of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the principle components andinterrelationships of the process control apparatus and system of oneembodiment of the invention; and

FIG. 2 is a graph illustrating a typical increase of solidsconcentration versus filtration time for a filter press dewatering unit.FIG. 2 further illustrates how a Resource Factor control signal isobtained as a measure of the rate of change of solids concentrationincrease with respect to time within the filter press.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A process control apparatus and system for separating a solids ladenedslurry and constructed in accordance with one embodiment of the presentinvention is shown in FIG. 1 and is indicated generally by the referencenumeral 11 in FIG. 1.

The system 11 shown in FIG. 1 incorporates a filter press 13 as theseparation unit, but (as pointed out above in the Summary of theInvention) some features of the invention are not limited to a filterpress type of separation unit but are instead equally applicable toother separation equipment, such as belt presses, centrifuges, andvacuum filters.

The objective of filtration with a filter press is to produce a filtercake which is substantially dry.

The dry solids concentration at the end of the filtration cycle can bedetermined by weighing a sample, heating the cake in an oven to driveoff the moisture then reweighing the sample.

Once the terminal dry solids of the cake is known, the dry solidsconcentration in the cake at various points in time (T_(i)) during thefiltration cycle in which that cake was formed can be back-calculated bythe following equation: ##EQU1## C_(i) =Intermittent cake concentration(weight % dry solids). V_(i) =Cumulative filtrate volume (liters) attime T_(i).

T_(i) =Time from start of filtration pumping into the filter press tosome point in time (i) (minutes).

T_(f) =Time from start of filtration pumping to end of filtrationpumping (minutes).

V_(p) =Volume of press (liters).

C_(f) =Cake solids concentration at time T_(f) (weight % dry solids).

V_(f) =Cummulative filtrate volume at time T_(f) (liters).

The following is an example of utilization of this equation:

    ______________________________________                                        Data                                                                                              V.sub.p = 1 liter                                                             C.sub.f = 40%                                             T.sub.i             V.sub.i                                                   Mins                Liters                                                    ______________________________________                                         0                    0                                                        2                   1                                                         4                   2                                                         6                   3                                                        12                   6                                                        18                   9                                                        30                  10.5                                                      45                  12                                                        60                  15                                                        90                  17.5                                                      120                 19                                                                            V.sub.f = 19 liters                                       ______________________________________                                         ##EQU2##

While this equation and method for determining the dryness orconcentration of cake solids in a filter cake is useful for backcalculating intermediate cake solids concentrations after the cake hasbeen formed (and after the final dryness of that particular cake hasbeen determined), it is not a really reliable formula or method fordetermining intermediate cake concentrations when the final cake drynessis not known. This formula and method assumes that there are noin-process changes during the cycle.

As a practical matter, the solids feed concentration (or percent ofsolids by weight in the influent slurry) can change substantially duringa cycle, or in the course of several filtration cycles. A heavy rain forexample can greatly effect the percent of solids in the influent slurry.

The present invention (as illustrated in FIG. 1 and as described in moredetail below) permits the constant readout of cake solids and solidsloading rate (pounds of dry solids per hour per square foot) contentwithin the filter press.

The governing factors are the filter press volume (V_(p)) in liters, thesludge feed solids concentration (C_(s)) expressed as percent dry solidsby weight, the sludge specific gravity (S_(s)), the dry solidsconcentration of the filtrate (C_(f)) expressed as percent dry solids byweight, the specific gravity of the filtrate (S_(f)), and the cumulativefiltrate volume in liters at a given time "i" (V_(i)) followingcommencement of filtrate.

The mathematical relationship is given as:

    C.sub.i =φ(V.sub.p, V.sub.i, C.sub.s, S.sub.s, C.sub.f, S.sub.f)

where C_(i) =filter cake solids concentration at time Ti (weight %, drysolids).

The mathematical expression is: ##EQU3##

The simplified assumption of 100% solids capture within the pressreduces the foregoing equation to the following form:

    C.sub.i =φ(V.sub.p, V.sub.i, C.sub.s, S.sub.f, S.sub.s)

which may be expressed as: ##EQU4##

A final simplified assumption of sludge feed specific gravity equal tothat of water provides the following formulization for cake solids:

    C.sub.i =φ(V.sub.p, V.sub.i, C.sub.s)

which is written as: ##EQU5##

Thus, in order to optimize filtration, the information that is needed todetermine the instantaneous percent solids in the filter press at anymoment in time (and to measure the rate of change of cake concentrationwith respect to time, the Resource Factor as defined in more detailbelow) is

(a) the percent of solids in the influent to the filter press,

(b) the measurement of the effluent cumulative volume at a given time

(c) the measurement of the effluent suspended solids concentration

(d) the volume of the filter press.

The present invention performs a continuous monitoring of the filtercake percent solids during the filtration cycle to insure that thepercent solids in the cake is the percent solids desired, even thoughthe percent solids in the influent to the filter press changes duringthe cycle.

In order to optimize filtration, in one specific embodiment of theinvention as shown in FIG. 1, a continuous monitoring of the incomingsludge dry solids concentration is performed by a sludge influentmonitoring system 15. The concentration (C_(s)) of the sludge isdetermined by photometric instrumentation such as the optronir modelsuspended solids analyzer manufactured by EDC/Enviro Development Co.,Inc., Mountain View, Calif.

The sludge influent monitoring system may also use other means fordetermining the concentration of dry solids in the incoming slurry. Forexample, sonar instrumentation may be used or radioactive techniques maybe used.

Monitoring system 15 produces a signal representing the solidsconcentration in the influent slurry, and this signal is transmitted online 17 to control system 19.

The influent slurry flows from the monitoring system 15 through a line21 and to a reaction zone 23.

A conditioning system 25 is, in the embodiment shown in FIG. 1,associated with the reaction zone by a line 27 so that the conditioningsystem can add conditioning agents to the influent slurry in thereaction zone 23 to enhance the dewaterability of certain types ofsludge (such as organic municipal sewage sludges) by structuring thesolids phase to permit ease of separation of the solid phase from theliquid phase.

In a preferred embodiment of the present invention, as illustrated inFIG. 1, the conditioning system 25 is connected to the control system 19by a control line 29 so that the amount of conditioning agent or agentsadded is regulated by a signal from the control system 19 which is basedon logical integration of one or more signals received by the controlsystem, including the percent solids concentration in the solidsconcentration signal in the influent slurry as transmitted to thecontrol system by the line 17.

The influent slurry is transmitted from the reaction zone 23 to a sludgesupply pump 31 through a line 33.

The sludge supply pump 31 is associated with the control system by acontrol line 35 which controls the output pressure and volume of thesupply pump 31.

The influent slurry is conducted from the output of the pump 31 to theinlet of the filter press 13 by a line 37.

The filter press 13 comprises (as described above in the introductorypart of this application) a filter press frame, filter cloths, filterplates and a hydraulic closing system. The filter plates form a seriesof cloth covered chambers into which the solids ladened sludge from theline 37 is pumped. The sludge is pumped to one side of the cloth in achamber and a relatively solid-free liquid is removed from the otherside of the cloth in the chamber. The hydraulic closing system keeps theplates in a closed filter pack which allows the dewatering of the sludgeunder pump supply pressure to occur. Thus, during the filtration cycle,dewatered sludge builds up on the sludge side of the cloth until a cakeis formed between adjacent filter plates.

The effluent from the filter press 13 is conducted through a line 41 toa filtrate monitoring system 43.

The filtrate monitoring system 43 measures the rate of flow of theeffluent from the filter press 13, and transmits a signal representingthis measurement to the control system 19 over a line 45.

The control system 19 with the measurement of calculated recumulativeflow versus time from the filtrate flow rate signal of the effluent (asreceived on the line 45) correlates the percent solids concentrationsignal of the influent slurry (as received over the line 17) and withthe known volume of the filter press 13 which is imputed by a thumbwheellocated in the control system 19 to provide a continuous monitoringsignal proportional to the solids concentration in the filter press 13.

In certain process applications this cake solids concentrations isdisplayed; in other process applications the monitored cake solidsconcentration is not displayed but it is further processed by processlogic circuitry for automatic control of one or more components of thesystem 11. In other applications the cake solids concentration signal isdisplayed and further processed to be a control signal.

Thus, in one specific embodiment, the continuously monitored solidsconcentration increase in the filter press 13 is used by the controlsystem 19 to stop the sludge supply pump 31 and to terminate the cycleautomatically when the desired solids concentration is attained.

In another specific embodiment of the invention as illustrated in FIG.1, the monitored solids concentration signal is further processed bylogic circuitry in the system 19 to regulate the conditioning system 25to obtain optimum feed ratios of conditioning agent to provide anacceptable sludge filtration. This permits economic optimization of theconditioning system, thus avoiding the costly waste of chemicals whichresults from the poor liquid/solids separation performance which resultsfrom over-conditioning and under-conditioning.

One preferred embodiment utilizing further processing of the signal bythe logic circuitry is the production of the Resource Factor which isthe rate of change of cake solids concentration within the filter press13 with respect to time.

The Resource Factor control signal is used by the control system toterminate the cycle under certain conditions.

For example, if the build up of the solids concentration is requiringexcessive time or is requiring an unacceptable amount of energy, theResource Factor control signal is used to terminate the cycle at acertain Resource Factor set point, even though the set point fortermination of the cycle at the desired solids concentration has notbeen reached.

Two conditioning agents that are commonly added to municipal sludge areferric chloride and calcium hydroxide.

The signal provided by the monitoring filtered-solids concentration inthe filter press 13 is used, in accordance with one embodiment of thepresent invention, to control the conditioning of the influent solidmixture of the slurry to either increase the amount of conditioningagents added if the sludge dewaters worse than designed or to decreasethe amount of conditioning agents added if the sludge dewaters betterthan design condition.

In some cases, this association between the solids concentrationincrease in the filter press and the conditioning system may also beused to increase the throughput by causing a faster build up of solidsin the filter press or to insure that the cake product meets certainrequirements, such as incineration or land fill requirements by assuringthat cake is produced which meets the required cake solidsconcentration.

A further embodiment of the present invention results from the use ofthe conditioning system 25 in some process applications, independentlyof the monitoring of the solids concentration increase in the particulardewatering unit. The conditioning system in this process applicationsmay be associated only with the sludge influent monitoring system 15 tovary the amount of conditioning agent or agents added in response onlyto changes in the influent slurry.

While not shown in FIG. 1, the monitoring of the final cake solidsconcentration in the filter press before discharge can also be used bythe control system 19 to feed a signal forward to a furnace in which thecake from the filter press is to be burned to provide improvedincineration of the cake solids. The control signal can be used tocontrol such furnace machine parameter as air flow to assure completecombustion.

FIG. 2 is a graph showing the increase of the filtered cake solidsconcentration versus the filtration time.

The resource factor is the rate of change of the slope of the curve at aparticular point on the curve, and the limiting set point for theminimum resource factor is indicated in FIG. 2 by the slope of the curveat the intersection of the line indicated by the legend RF_(min).

At a given point in the filtration cycle, the rate of increase of cakesolids concentration with respect to time (expressed as the derivativedC_(i) /dT and termed the resource factor as noted above) does notjustify the continued expenditure of energy to operate the dewateringsystem.

The minimum resource factor will depend on many factors including thefollowing factors of the sludge: type, nature of solids, chemicalcomposition, concentration, conditioning. The selection of a minimumresource factor further depends on the use of the sludge cake (e.g.,whether land fill or incineration). The value of the minimum resourcefactor is determined experimentally for each sludge or for class ofsludges described by factors such as listed earlier in this paragraph.Once established the resource factor is entered into the control system19 by a device such as a thumb wheel and is referenced in the controllogic. The resource control can also be used to control for a dewateringcondition in which the dewatering is better than designed, and theresult is to get a greater production of cake solids than the designcondition. In this embodiment, the control of the cycle time is solelyby the minimum and the maximum resource factor set points withoutterminating at a given solids concentration increase set point.

In another specific embodiment of the present invention, the controlsystem 19 controls the sludge supply pump 31 during thebuild-to-terminal pressure stage to maintain a constant effluent flowrate, out of the filter press 13 which is measured by the filtratemonitoring system 43. This constant effluent flow rate maximizes theefficiency of the increase of the cake solids in the filter press forcertain sludge which can be generally classified as incompressible.

In this embodiment the flow rate signal on the line 45 is used by thecontrol system 19 to control the output of the supply pump 31 tomaintain the constant effluent flow rate during the build-to-terminaland maximum pumping pressure. After the terminal pumping pressure isreached, the pump 31 continues to pump at that pressure until the end ofthe cycle.

The following example compares the present invention pumping systemwhich maintains a constant effluent flow rate with a prior art pumpingsystem of the kind in which the supply pump 31 was used to fill thefilter press rapidly at virtually maximum, uncontrolled output of thesupply pump 31 during the build-to-terminal pressure stage.

In the following example the reference letter A indicates the presentinvention and the reference letter B indicates the prior art pumpingsystem.

EXAMPLE

Sludge Type: Iron and Copper Hydroxide Sludge

Feed Solids Concentration=4.4% Dry Solids

Press Volume 0.1104 ft.³ =2.94 liters

    ______________________________________                                                          Cumulative Cake                                                     Pressure  Filtrate   Solids  Resource                                 Time    psi       Flow, Liters                                                                             %       Factor                                   MINUTES A      B      A     B    A    B    A    B                             ______________________________________                                        0        0      0     0     0    0    0    0.63 0.51                          15      125     50    6.22  4.5  9.5  7.7  0.29 0.25                          30      186     92    10.5  8.1  13.9 11.5 0.21 0.20                          45      195    101    13.5  11   17.1 14.5 0.17 0.15                          60      204    110    16    13.2 19.7 16.8 0.15 0.13                          75      204    190    18.2  15.8 22.0 19.5 0.13 0.14                          90      215    222    20.0  17.8 23.9 21.6 0.13 0.14                          105     220    225    22.0  19.8 25.9 23.7 0.13 0.14                          120     224    222    23.5  21.5 27.5 25.4 0.11 0.11                          135     225    219    25.0  23.0 29.1 27.0 0.11 0.11                          ______________________________________                                         A = present invention constant filtrate flow                                  B = State of the Art rapid fill pumping system                           

The benefits of the present invention can be illustrated in the lastline under the caption "Cake Solids" where it is seen that the presentinvention provides greater cake solids than the prior art pumping systemin the same amount of time. Alternatively, the present invention can beused to obtain the same amount of cake solids as the prior art pumpingsystem but in a shorter period of time than was required by the priorart pumping system.

While we have illustrated and described the preferred embodiments of ourinvention, it is to be understood that these are capable of variationand modification, and we therefore do not wish to be limited to theprecise details set forth, but desire to avail ourselves of such changesand alterations as fall within the purview of the following claims.

We claim:
 1. A process control method of dewatering a solids ladenedslurry of water and waste water in a filter press dewatering unit of thekind in which the solids are removed from the slurry and concentrated inthe filter press in dewatering cycles, said method comprising,feeding aninfluent slurry ladened with relatively incompressible solids to aninlet of the filter press, removing the solids from the slurry in thefilter press and concentrating the removed solids within the filterpress, conducting a relatively solids-free effluent from the filterpress, controlling the pumping of influent into the filter press tomaintain a substantially constant flow rate in the effluent from thefilter press until the terminal pump pressure of the cycle is reached tothereby maximize the increase of solids within the filter press duringthe build-to-terminal pressure phase of the pumping operation,generating a signal characteristic of said terminal pump pressure, andterminating operation of said pump in response to said signal.
 2. Theinvention defined in claim 1 including continuously monitoring thesolids concentration increase in the filter press and terminating thedewatering cycle when the monitored solids concentration in the filterpress reached a certain amount.
 3. The invention defined in claim 2including measuring the rate of change of cake solids increase withrespect to time,producing a resource factor control signal from themeasured rate of change of cake solids increase, and terminating thedewatering cycle at a certain minimal resource factor control signal setpoint even though the monitored solids concentration in the filter presshas not reached said certain amount.
 4. A process control apparatus andsystem for dewatering a solids ladened slurry of water and waste waterin a filter press dewatering unit, said system comprising,a filter presshaving an inlet, filter cloth chambers for removing solids from theslurry and for concentrating the removed solids in filter cakes withinthe filter cloth chambers, and an outlet, influent feed means forfeeding an influent solids ladened slurry to the inlet of the filterpress, effluent means for conducting a relatively solid-free effluentfrom the filter press, pump means for pumping the influent solidsladened slurry under pressure through the influent feed means to thefilter press, pump control means for controlling the pumping of influentinto the filter press to maintain a substantially constant flow rate inthe effluent from the filter press until the terminal pump pressure ofthe cycle is reached to thereby maximize the increase of solids withinthe filter press during the build-to-terminal pressure phase of thepumping operation, means to generate a signal characteristic of terminalpump pressure, and means to terminate operation of said pump in responseto said signal.